1. Field of the Invention
The present invention relates to an electromagnetic driving device, in which a movable core is supported in a housing portion which co-operates with the movable core to form an electromagnetic circuit, and a flow rate controlling apparatus employing the electromagnetic driving device.
2. Description of the Related Art
In an electromagnetic driving device in which an electromagnetic circuit is constituted by a movable core, a housing portion for housing therein the movable core in such a manner as to move in a reciprocating fashion and an attracting portion for attracting the movable core in one of reciprocating directions by virtue of magnetic attraction, it is known, as a means for supporting the movable core in such a manner as to move in the reciprocating fashion, to carry a shaft press fitted in the movable core on a bearing or to support the movable core with a plate spring. With these supporting means, since the housing portion and the movable core do not slide on each other and therefore the resistance value of, for example, friction generated by the reciprocating movement of the movable core is not affected by the value of electric current that is supplied to a coil of the electromagnetic driving device, the reciprocating positions of the movable core remain substantially constant both in increasing and decreasing directions of the value of electric current that is supplied to the coil in case the current value remains the same. Namely, the hysteresis of the reciprocating positions of the movable core is small relative to electric current that is supplied to the coil.
However, since the movable core is supported by the housing portion which houses therein the movable core in such a manner as to reciprocate and co-operates with the movable core for formation of an electromagnetic circuit and the bearing or plate spring which is a separate member, there may occur a case in which the axial centers of the housing and the movable core deviate from each other. If the axial centers of the housing and the movable core deviate from each other, the movable core slides on the housing portion locally, whereby there may be a risk of imbalanced wear of the movable core. In order to prevent the occurrence of imbalanced wear due to such an axial center deviation, it is a general practice to provide a larger gap between the movable core and the housing portion in a radial direction which is normal to the direction in which the movable core reciprocates so as to prevent the movable core from sliding on the housing portion locally. However, with the larger gap formed between the movable core and the housing in the radial direction, the magnetic flux which flows through the magnetic circuit constituted by the housing portion, the attracting portion and the movable core, lowers, and an attracting force acting between the movable core and the attracting portion lowers in turn. Then, if the number of turns of the coil is increased in order to prevent reduction in the magnitude of the attracting force, there is caused a problem that the size of the electromagnetic driving device is enlarged.
With a view to solving the problem, it is considered that the gap formed radially between the housing portion and the movable core should be made as small as possible by allowing the movable core to be supported directly on the housing portion. Even in the construction in which the housing portion directly supports the movable core, in order for the movable core to slide smoothly, a gap is formed in a radial direction between an inner circumferential wall of the housing portion and an outer circumferential wall of the movable core (hereinafter, the xe2x80x9cgap formed in the radial direction between the inner circumferential wall of the housing portion and the outer circumferential wall of the movable corexe2x80x9d is referred to as a side gap). However, if the movable core becomes eccentric relative to the housing portion when the movable core reciprocates, there is generated a portion where the side gap becomes narrow locally, and a force with which the movable core is attracted in the radial direction by the housing portion (hereinafter, the xe2x80x9cforce with which the movable core is attracted in the radial direction by the housing portionxe2x80x9d is referred to as a xe2x80x9cside forcexe2x80x9d) becomes large.
Furthermore, a sliding resistance generated between the housing portion and the movable core increases as the value of electric current supplied to the coil increases. Therefore, the hysteresis of the reciprocating positions of the movable core relative to the electric current value becomes large in increasing and decreasing directions of the value of electric current that is supplied to the coil. For example, in a case where an electromagnetic driving device is used in a flow rate controlling apparatus, there is caused a problem that the flow rate or the fluid pressure becomes different between the increasing direction and the decreasing direction of electric current.
An object of the present invention it to provide an electromagnetic apparatus in which a movable core is supported by a housing portion, in which the hysteresis of reciprocating positions of the movable core is small relative to the value of electric current that is supplied to a coil and which can be miniaturized, and a flow rate controlling apparatus employing the same driving device.
With any of an attracting portion, a housing portion and a movable core being set to become magnetically saturated with an electric current value which exceeds 60% of the maximum value of electric current that is supplied to a coil, a large side force acts between the housing portion and the movable core as the value of electric current that is supplied to the coil increases and approaches the electric current value at which the magnetic saturation occurs. In particular, when the movable core becomes eccentric relative to the housing portion so that a side gap becomes partially narrow or the movable core is brought into contact with the housing portion the side force increases drastically. Thus, when the side force increases there occurs a large resistance acting between the housing portion and the movable core. Consequently, the hysteresis of the reciprocating positions of the movable core increases relative to the electric current value in increasing and decreasing directions of electric current that is supplied to the coil.
When used herein, that a certain member is saturated magnetically means a state in which the magnetic flux density thereof increases to be so dense that no more magnetic flux is allowed to flow through the member. A state is not referred to as being magnetically saturated in which only part of the member such as a corner portion thereof becomes saturated but move magnetic flux can flow through the remaining portion of the member.
In addition, if any of the attracting portion, the housing portion and the movable core becomes saturated magnetically with an electric current value which is smaller than 40% of the maximum electric current value, the attracting force acting between the movable core and the attracting portion does not increase and therefore, in order to obtain a required travel, the number of turns of the coil must be increased.
Then, to cope with this, according to a first aspect of the present invention, there is provided an electromagnetic driving device in which any of an attracting portion, a housing portion and a movable core is set to become magnetically saturated when the value of electric current that is supplied to a coil increases to become a predetermined value which is between 40% or larger and 60% or smaller than the maximum value of electric current so supplied to the coil. Since magnetic saturation occurs when the electric current value reaches the predetermined value which is equal to or smaller than 60% of the maximum electric value, even if the movable core is eccentric relative to the housing portion, the upper limit value of the side force acting between the housing portion and the movable core can be reduced so that the upper limit value of sliding resistance acting between the housing portion and the movable core can be reduced in turn. Consequently, the hysteresis of the reciprocating positions of the movable core becomes small relative to the value of electric current that is supplied to the coil.
Furthermore, since there occurs no magnetic saturation until the value of electric current that is supplied to the coil reaches the predetermined value which is equal to or larger than 40% of the maximum electric value, a magnetic attraction force can be secured which is required to attract the movable core for displacement.
According to a second or fourth aspect of the present invention, there is provided an electromagnetic driving device wherein a non-magnetic layer is formed on at least one of the sides where the housing portion and the movable core are situated, respectively, to diametrically face each other, the total thickness of the non-magnetic layer so formed being between 40 xcexcm or larger and 80 xcexcm or smaller. Even if the movable core becomes eccentric relative to the housing portion, so that a side gap formed between the movable core and the housing portion becomes narrow locally, at least a magnetic gap equal in magnitude to the thickness of the non-magnetic layer so formed can be securely formed between the magnetic materials of the movable core and the housing portion. Consequently, even if the movable core becomes eccentric relative to the housing portion, the upper limit value of the side force acting between the housing portion and the movable core can be reduced so that the upper limit value of sliding resistance acting between the housing portion and the movable core can be reduced in turn. Consequently, the hysteresis of the reciprocating positions of the movable core becomes small relative to the value of electric current that is supplied to the coil.
According to a third, fifth or sixth aspect of the present invention, there is provided an electromagnetic driving device wherein the hardness of at least one of sliding surfaces of the housing portion and the movable core is made equal to or larger than HV200. The sliding resistance between the movable core and the housing portion at the sliding portion can be reduced by increasing the hardness of the sliding surface. Consequently, even if the movable core becomes eccentric relative to the housing portion, the upper limit value of the side force acting between the housing portion and the movable core can be reduced so that the upper limit value of sliding resistance acting between the housing portion and the movable core can be reduced in turn. Consequently, the hysteresis of the reciprocating positions of the movable core becomes small relative to the value of electric current that is supplied to the coil. Furthermore, since a difference in hardness between the sliding surfaces is set to be equal to or smaller than HV300, even if the movable core is forced to slide on the housing portion, the component which is smaller in hardness can be prevented from wearing in an imbalanced fashion.
According to a seventh aspect of the present invention, there is provided a flow rate controlling apparatus with an electromagnetic driving device according to any of the first to sixth aspects of the present invention. Consequently, a lower hysteresis can be provided due to a relationship between the increasing and decreasing directions of electric current and the reciprocating positions of the movable core. Consequently, the flow rate or pressure of the fluid can be controlled with high accuracy relative to the value of electric current that is supplied to the electromagnetic driving device.
The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.